In order to achieve a color display, color filters are often used. The most common approach is to add color filters on top of three black/white sub-pixels of a pixellated display to display the red, green and blue colors. When a red color is desired, the green and blue sub-pixels are turned to the black state so that the only color displayed is red. When a blue color is desired, the green and red sub-pixels are turned to the black state so that the only color displayed is blue. When a green color is desired, the red and blue sub-pixels are turned to the black state so that the only color displayed is green. When the black state is desired, all three-sub-pixels are turned to the black state. When the white state is desired, the three sub-pixels are turned to red, green and blue, respectively, and as a result, a white state is seen by the viewer.
The disadvantage of such a technique is that the white state is fairly dim. To compensate it, a fourth sub-pixel may be added which can display only the black and white states. In this approach, the white level may be increased, but the increase in whiteness is at the expense of the red, green or blue color level (where each of the sub-pixels is only one fourth of the area of a pixel). In other words, a brighter white color can be achieved, but it is achieved at the expense of the other colors which will become light and unsaturated. Even with this approach, the white level is normally still substantially less than half of that of a black and white display, rendering it an unacceptable choice for display devices, such as e-readers or displays that need well readable black-white brightness and contrast.